Cygnus X-1: shedding light on the spectral variability and polarization of black holes

Abstract

We present an analysis of extensive recent monitoring observations of the black hole HMXB Cygnus X-1 obtained as part of the 2007 to 2011 Key Programme (KP) observations of the INTEGRAL mission. Cyg X-1 is one of only three persistent black hole binaries in our galaxy that spend most of their time in the hard spectral state. After spending 3 years in the hardest regime of its parameter space, the source displayed a softening and flaring episode in mid 2009, entered a soft state in early 2010 June and entered a transitional phase in April 2011. While the hard X-ray spectrum of Cyg X-1 is one of the best studied examples of its kind, e.g. through our monitoring campaign with RXTE and coordinated radio observations with AMI (formerly: Ryle) telescope, the INTEGRAL monitoring allows us to study the spectral evolution from about half an hour over a few days to weeks, timescales that have been only sparsely sampled so far. The parameter ranges for the hard and soft states as well as the transitional phases are constrained. These measurements are of special importance for understanding the physics of the hot plasma of the jet base and/or the corona, but typically difficult to obtain for transient source. Additionally, we report on polarization studies which have been conducted using a larger INTEGRAL/IBIS dataset and led to the discovery of gamma-ray polarization in Cyg X-1.